Description from Flora of China
Herbs perennial, small. Taproot long-conic, rarely fusiform. Stem simple, usually very short and appearing acaulescent. Basal leaves petiolate, petioles sheathing. Leaves reduced upwards. Inflorescence branching, umbels compound, terminal on stem and branches; bracts entire, 2–3-lobed to pinnate or absent; rays 5–20, those of primary terminal umbel stout, spreading-ascending or diffuse; bracteoles similar to bracts or absent; umbellules 10–30-flowered. Calyx teeth usually minute or obsolete. Petals ovate or obovate, white or purplish-red, base cuneate or shortly clawed, apex incurved and notched. Stylopodium low-conic; styles spreading to reflexed. Fruit broadly ovoid, rarely oblong-ovoid, slightly laterally compressed, glabrous, sometimes with small tubercles between ribs; ribs filiform, conspicuous; vittae 1–4 in each furrow, 4–8 on commissure. Seed face slightly to deeply concave. Carpophore various.
This genus has received very mixed treatments since its establishment by Lindley in 1835. Norman (J. Bot. 76: 229–233. 1938) studied the genus critically, resolving some of the confusion, and commented that probably all plants assigned to Trachydium since Lindley’s day really belong to other genera. However, the high-altitude, dwarf plants exhibit complex variation and the taxonomy continues to be controversial, particularly when delimiting boundaries with other genera containing similar species of reduced stature (e.g., Aulacospermum, Chamaesciadium, Chamaesium, Ligusticum, Physospermopsis, Pleurospermum, Schulzia, and Sinocarum). Some authors follow Norman and limit Trachydium to a unispecific genus including T. roylei, while others extend the circumscription to include anything up to 14 additional species. It is acknowledged that the following species form a heterogeneous group, but a conservative treatment has been adopted here as new, comprehensive material is needed to determine proper specific and generic limits.
The following species have been described from Chinese material, but are imperfectly known as no specimens have been seen or the specimens are inadequate.
Trachydium szechuanense H. Wolff (Acta Horti Gothob. 2: 299. 1926), described from N Sichuan (ca. 4000 m, K. A. H. Smith 3428, holotype, GB).
Trachydium variabile H. Wolff (Acta Horti Gothob. 2: 298. 1926), described from N Sichuan (4000–4500 m, K. A. H. Smith 2988, 3205, 3232, 3723, 3888 & 4181, syntypes, GB).
About six species (see following note): widespread across C Asia to the Himalayan region and SW China; six species (four endemic) in China.
(Authors: Pu Fading (溥发鼎 Pu Fa-ting); Mark F. Watson)